Life Cycles of Stars

How Stars are Born

No one has ever observed stars go through their life cycle, but astronomers can observe
many stars at different stages in their life cycles. In addition, astronomers can
calculate what would happen to stars under various conditions, and attempt to match the
predictions against actual stars. The overall outlines of stellar evolution are probably
accurately known, but there are many unanswered questions, some of them big ones, and very
likely there are surprises waiting for astronomers as well.

Stars are believed to form when clouds of interstellar gas and dust start to contract
under the influence of gravity. In interstellar space, far from other stars, a cloud of
gas can be very thin and still be dense enough to begin contracting. Many astronomers
believe violent explosions of older stars create shock waves that help start the
contraction process, at the same time enriching the gas in heavy chemical elements. In all
likelihood, a cloud will condense into many stars and form a star cluster.

Initially, a cloud destined to become a star will be roughly spherical. The cloud will almost
certainly have a slight rotation, simply because of random gas motions in the cloud as it
started contracting. As the cloud contracts, its rotation will speed up, causing the cloud
to become disk-shaped. The cloud is still far larger than our solar system, and its outer
reaches very thin. Most of the mass of the cloud falls to the center, adding energy to the
center and heating it up. The center of the cloud becomes a protostar, emitting
mostly infrared radiation. Finally the temperatures and pressures within the star reach
the point where nuclear reactions begin, and the star "turns on". Matter in the
surrounding disk may accrete to form planets, or, if the condensations are massive enough,
companion stars.

Stars With Companions

Multiple Stars

Many, perhaps all stars, have companions. Many stars are binary stars or multiple
stars, with several stars orbiting around one another. Some multiple stars may form
when a rapidly rotating protostar becomes unstable and splits; others may form as separate
protostars. Our nearest neighbor, Alpha Centauri, is a triple star, with a primary
star much like our sun, a smaller and cooler secondary star orbiting about as far
away as Uranus orbits our sun, and a very faint companion many times farther away from the
primary than pluto is from the sun. Binary stars are of great value to astronomers because
the orbital periods of binary stars depend on the masses of the stars, and binary stars
enable astronomers to determine the masses of stars

Does the sun have a companion star? There has been speculation from time to time that
the sun might have a companion, most recently the "nemesis" hypothesis. The
"nemesis" hypothesis argued that an undiscovered companion of the sun
periodically caused comets to sweep through the solar system, triggering among other
things the extinction of the dinosaurs. If any undiscovered companion exists, it must be
very faint, not very massive, and very far away. The odds are very much against such a
companion escaping detection this long.

Planetary Systems

The sun has planets. From what we know of the formation of the solar system, it seems
very likely that many if not most solitary stars have planets, formed from the material
that did not condense into the central protostar. Possibly widely-separated multiple stars
like alpha centauri have planets as well. Whether exotic multiple stars with many
components, or very close binary systems can have planets is uncertain. Even if planets
form around such stars, close encounters with the stars may fling the planets into a star
or out of the star system altogether. A number of stars have disks of solid matter
orbiting them that are believed to be similar to the disk from which the planets of our
Solar System formed. Such disks have been termed proplyds (Protoplanetary
Disks).

Locating other planetary systems is a very great challenge. One approach is to detect
very tiny, regular variations in the positions of stars. As a planet orbits a star, the
star and planet both orbit around their center of mass; if the star and planet were joined
by a beam, the point where the beam would balance is the center of mass. Tiny motions of
the star can also be detected using the Doppler Effect. The motions are very tiny: it
would be hard to detect the effect of Jupiter on the sun from a nearby star. Another
approach is to block the light of the star and search for the reflected light from
planets. All of these techniques are extreme challenges to existing technology but are
becoming more feasibile as instruments improve. Beginning in 1993 evidence for other
planetary systems has rapidly accumulated. Some of these newly-discovered objects are so
massive they may be so-called brown dwarfs, on the borderline between large planets
and tiny stars.

The newly-discovered objects around other stars are very unusual. None of the probable
other solar systems look much like our own. Many of the planets are much more massive than
Jupiter and some of these objects orbit surprisingly close to their suns. It appears that
our solar system may be unusual.

Are there objects midway between planets like Jupiter and small stars? The fainter
stars are, the more numerous, so that we might expect such objects, nicknamed brown
dwarfs to be very common. None have been conclusively detected, yet, even though
their infrared radiation would be easily detectable. It appears that there is a sharp
dividing line between solitary and multiple stars, for reasons still unclear. Some of the
newly-discovered objects around other stars are so massive they may be brown dwarfs.

Evolution of Stars

Life on the Main Sequence

Once stars begin to shine, they assume a position on the main sequence and tend to stay
there, shining steadily. Stars like the sun would brighten somewhat in their first couple
of billion years. The brightening of the sun poses a problem called the faint early
sun problem: geological evidence indicates that the earth has been warm enough to
have liquid water throughout its history, yet the early sun was perhaps 25% less bright
than the present sun, and the early earth should have been cold. Perhaps the early earth
had a denser atmosphere that trapped heat more than the atmosphere does now. Both
geologists and astronomers are actively pursuing research on this question.

Almost every aspect of the life of a main sequence star is determined by one fact: its
mass. Very tiny stars emit light feebly and remain cool. Such stars are called red
dwarfs. More massive stars are hotter and brighter. Massive stars have more fuel to
sustain their output, but their energy output or luminosity is roughly proportional to the fourth
power of their mass. A star twice as massive as the sun will emit energy 2 x 2 x 2 x 2 or
16 times the rate of the sun. With only twice as much fuel to sustain it, the star will
only shine 2/16 or 1/8 as long as the sun before running out of fuel.

We might compare stars to people. Red dwarf stars spend their energy frugally, like
someone with a very limited income, whereas bright, massive blue-white stars run through
their fuel quickly, like a lottery winner on a spending spree. Red dwarf stars can last
tens or hundreds of billions of years, doling out their fuel at a miserly rate. Our own
sun will shine for perhaps 10 billion years, but bright blue-white supergiants like
Deneb
or Rigel might last only a few million years.

Leaving the Main Sequence

A rough estimate of star lifetimes on the Hertzsprung-Russell diagram is
shown below. The hottest stars on the Main sequence don't even last a million
years, but red dwarfs last far longer than the age of the Universe. Stars,
whether in Hollywood or the sky, follow the principle "Live hard, die young,
leave a good-looking corpse."

When main sequence stars run out of available fuel, the nuclear reactions in the center
of the star die out. Without the intense radiation pressure produced by these nuclear
reactions, the star begins to contract under its own gravity. As matter falls in toward
the center of the star, it releases energy that heats the star until finally a new
sequence of nuclear reactions begins. The renewed energy output heats the outer part of
the star, causing it to expand. At this point the star leaves the main sequence. The outer
layers expand and cool, causing the star to redden, but the enormous size of the star
gives it a vast surface area through which it emits a tremendous amount of energy. The
star becomes a red giant or supergiant.

The most massive stars leave the main sequence soonest. We can see the evolution of
stars clearly by plotting H-R diagrams for star clusters, whose members all formed at the
same time. Very young clusters may even have remnants of their parent gas clouds still
visible and contain nothing but main sequence stars, and even stars that have not yet
reached the main sequence. Older clusters have some of their brightest stars leaving the
main sequence, and in very old clusters, most of the stars brighter than the sun have left
the main sequence. The evolution of the H-R diagram of a star cluster is rather like
peeling a strip off a banana; as the star cluster ages, the giant star branch becomes
larger and the branching point moves farther down the main sequence.

Since the lifetimes of Main Sequence stars increase rapidly with decreasing
brightness, H-R plots of star clusters commonly have a Giant Branch peeling off
in classes O, B, and A. Only old clusters will have a Giant Branch peeling off
at F, and only the most ancient globular clusters are old enough to have G stars
enter the giant phase. The Universe isn't old enough yet for K stars to have
gone giant. M and cooler stars are not massive enough to collapse and begin
helium fusion, and they will never become giant stars.

The Fates of Stars

Gravity and the Collapse of Stars

All objects in the universe exist because of a balance between gravity and some
counteracting force. Four fundamental forces of nature (gravity, electromagnetism, weak
nuclear force and strong nuclear force) govern the structure and bonding of atoms. It is
fitting that we return to these same basic forces here when we examine matter on the
largest scale.

Left to itself, gravity would pull all masses together to a central point. In planets,
the counteracting force is the atomic bonding between atoms, and the repulsion between the
negatively-charged electrons of neighboring atoms. In normal stars, the counteracting
force is the thermal motion of the atoms in the hot gas of the star, and the outward
pressure exerted by radiation.

When the radiation pressure in a star falters, gravity begins pulling the gas of the
star inward. As the gas falls inward, it gains energy and heats up the interior of the
star. Also, the gas becomes more tightly compressed and the pressure increases. Several
things can happen, depending on the mass of the star. The temperature and pressure inside
the star can rise until a new generation of nuclear reactions begins, the star can
continue to collapse until some new counteracting force stops the collapse, or the star
can collapse until gravity actually does pull all the mass of the star into a single
point: a black hole. The more massive a star is, the more dramatic its end.

The Fate of Very Small Stars

Small, faint, red dwarf stars probably never do anything very dramatic. They continue
to fuse hydrogen to helium at a miserly rate. Even nearby red dwarf stars are very faint.
If there were none within a few dozen light years of earth we would not know they exist at
all, but judging from what we see in the space near the sun, red dwarfs are among the most
common stars. These stars can continue to shine, if one can use that word for such faint
stars, for tens or hundreds of billions of years, gradually cooling as their fuel runs
out. Their mass is so small they will never collapse enough to start a new cycle of
activity. The atomic repulsion between atoms will counteract gravity. Even then, their
surface area is so small they will remain warm for a very long time.

Average-Sized Stars, Red Giants, and White Dwarfs

Stars ranging from 10 per cent to several times the mass of the sun go through a
different final history. As the star's hydrogen supply begins to run out and its energy
output falters, the star begins to contract. As the matter of the star falls inward, it
acquires energy and the star heats up. Eventually, the temperature and pressure inside the
star get high enough that the helium in the star can begin to fuse to make carbon. The
core of the star is very dense, and its energy output heats the outer gases of the star,
causing them to expand. The star swells enormously, becoming perhaps as large in diameter
as the earth's orbit: 300 million kilometers (186 million miles). A red giant consists of
a dense core and a vast but very thin outer atmosphere. It has a huge surface area to
radiate energy, so red giants are very luminous, but the energy is spread thinly. The
surface temperature of the star is low, which is why red giants are red.

Many red giants are unstable. Instead of swelling to a given size and maintaining it,
the stars pulsate and vary in brightness. Some red giants pulsate rhythmically, others
flare up in irregular bursts. Red giants include many varieties of variable stars.

Red giant stars also shed matter into space. Many giants shed matter steadily, others
violently. In their later life cycles, some pulsating giants eject great shells of matter
which form luminous envelopes around the star. Because these gas envelopes look disk-like
in a telescope, they are called planetary nebulae.

Some giants are massive enough to start other cycles of nuclear reactions after their
helium runs out; they fuse carbon and perhaps even heavier elements, but sooner or later
all red giants run out of nuclear fuel. Their thin outer envelopes are ejected into space
or gathered up into their core. The core collapses until a new counteracting force comes
into play. The electron shells of the atoms are crushed out of existence and the electrons
wander between densely-packed atomic nuclei. The forces between these electrons prevent
the star from collapsing further. By this time, the star may be only about as large in
diameter as the earth, even though it is as massive as the sun. The matter of the star,
called degenerate matter, is so dense that a teaspoonful would weigh many tons on
earth. This final stage of the star is a white dwarf. White dwarfs are very hot,
but their surface area is so small that they are very faint and lose their heat very
slowly. The coolest known white dwarf is about 3900 K. The Universe is not old
enough for any white dwarf stars to have cooled completely.

The End of the Sun

In all likelihood, the sun will become a red giant. In about 10 billion years, all the
hydrogen in the core of the sun will have been used and the sun will start to contract
under its own gravity. It will heat up and brighten as it does, probably making the earth
too hot for life. When helium begins to fuse in the sun's core, the outer gases of the sun
will expand, probably enveloping the earth. The gas will be hot, but very thin, and for
several thousand years the earth may actually orbit within the star, slowly heating up by
contact with the thin hot gas, and eventually being destroyed as friction with the gas
causes the earth to spiral into the hotter interior of the star. Eventually the sun will
eject its outer envelope, or absorb it, leaving only its core as a white dwarf. In the
unlikely event the earth survives the red giant stage, the final white dwarf will emit
only a fraction of the present energy of the sun and the earth will be frozen solid.

Evolution of Multiple Stars

If binary or multiple stars are far apart, they will evolve independently of one
another. However, astonishing things happen when multiple stars are close together. When
one of the stars reaches the red giant phase, it can swell large enough to exchange gas
with its partner star. What happens depends on the ages of the two stars, their masses,
and how rapidly they exchange matter.

If the partner star is a white dwarf, some very violent events can happen. White dwarfs
have used up all their nuclear fuel. If a large amount of hydrogen falls onto a white
dwarf, it can undergo nuclear fusion right on the surface of the star. The resulting
outburst will cause the star to brighten briefly by hundreds of times, becoming a nova
(Latin, new). Even more dramatic outbursts are possible: the white dwarf can
accumulate mass until its internal pressures become great enough for the next generation
of nuclear reactions begin. When that happens, the renewed nuclear activity will blast off
the outer layers of the white dwarf, creating a type I supernova.

Massive Stars and Supernovae

Very massive stars have a more dramatic end yet: they become Type II Supernovae,
stars that explode and briefly outshine all the other stars in the galaxy combined.
Because old stars lose mass in the red giant stage, it is hard to predict exactly which
stars will meet this most violent of fates.

Red giants with 20 or so times the mass of the sun develop cores with a concentric
shell structure. Each shell is hotter and denser than the one outside. In the outermost
shell, hydrogen fuses to helium as in any normal star. In the next shell in, helium fuses
to carbon. Succeeding shells within fuse carbon, oxygen, neon, and silicon until, at the
center, there is a core of inert iron. Iron cannot yield energy by fusing to make heavier
atoms, so this innermost core is the end product of fusion in the star.

At first glance, it looks as if the star can turn entirely to iron, because each shell
uses up the residue from the shell outside it. But there is a catch: eventually the core
becomes so massive that it cannot withstand the pressure of gravity. The core collapses
until a new counteracting force halts the collapse. The only force capable of halting the
collapse is the strong nuclear force. The core of the star collapses until it becomes, in
effect, a gigantic atomic nucleus. The core becomes a neutron star. It may have
more mass than the sun but be only 15 kilometers (10 miles) across. A teaspoonful of this
matter would weigh thousands of tons on earth.

Where there was once the hot, dense core of the star, there is briefly a void. The
neutron star core, more massive than the sun but not much bigger than a mountain on earth,
has an incredible gravitational pull. At a distance of 10,000 kilometers (6,000 miles) a
neutron star as massive as the sun exerts a gravitational pull about 1,300 times stronger
than that on the surface of the earth. In this enormous gravitational field, the matter of
the star falls inward, reaching perhaps a tenth of the speed of light.

The results are, to say the least, impressive. Several times the mass of our sun
crashes into the surface of a neutron star at up to a tenth of the speed of light. This
gas is heated and compressed beyond anything we can imagine. We can do the calculations
and write the numbers, but nobody can really comprehend the titanic amount of energy
involved. Nuclear reactions run rampant; there is enough energy available and a high
enough density of fast-moving particles to build nuclei as heavy as plutonium and probably
far heavier. The neutron star core itself is almost incompressible even under these
conditions, and the impacting matter rebounds as a shock wave. This event, called the core
bounce, tears the star apart. In a few hours, the shock wave reaches the surface,
tearing off the outer layers and exposing the hot interior of the star. The star brightens
to billions of times its normal brightness.

Historical Supernovae

Supernovae are commonly seen in distant galaxies. If our own galaxy has supernovae as
often as other galaxies, there is probably one every few years. Yet only a dozen or so
supernovae in our own galaxy have been witnessed from earth. The rest have been obscured
by gas and dust clouds in our galaxy.

The supernova of 1572 was brighter than Venus and could be seen in broad daylight, even
though it was 5000 light years away. Another supernova occurred in 1604. These events came
at a critical time, just as astronomers were beginning to question the ancient notion that
the heavens were perfect and unchanging. A supernova in 1006 rivaled the moon in
brightness. Most supernovae have absolute magnitudes of about -21: at a distance of 32.6
light years it would shine at magnitude -21 and far outshine the moon. In 1885, a
supernova in the Andromeda galaxy reached magnitude 7. Across 2.2 million light years,
that single star was almost bright enough to see with the unaided eye. All these events,
however, happened before astronomers had the observing techniques to study supernovae in
detail.

The Supernova of 1987

Almost four centuries of not-very-patient waiting ended in 1987 when the first
supernova since 1604 visible to the unaided eye appeared. The supernova occurred not in
our galaxy, but in the Small Magellanic Cloud, a small satellite galaxy of our own about
180,000 light years away.

Almost from the beginning, supernova 1987 followed a script all its own. It only
reached magnitude 3, not the magnitude 1 that astronomers expected, but it stayed at peak
brightness much longer than most supernovae. Most paradoxical of all, the star that
produced the supernova was not an aging red supergiant, but a seemingly stable blue-white
supergiant. It now appears that some red supergiants can lose their outer envelopes
quietly, revealing their hot interiors more clearly.

Life (Briefly) Near a Supernova

What might it be like on a planet orbiting a star that went supernova? Imagine the
planet receives as much radiation as we get from the sun. It is hard enough to imagine the
energy output of the sun, let alone a supernova, so let us scale things down a bit first
by asking what it would take to match the sun's output at a distance of only one
kilometer. The sun emits 77 megatons of energy every second, but it is 150 million
kilometers away, and the intensity of radiation drops off as the square of the distance.
To find out what energy output we need for a distance of one kilometer, we must divide the
sun's output by the square of its distance. The answer comes out to about .0000035
megatons, or the equivalent of 3.5 tons of high explosive. It is not hard to picture an
explosion of 3.5 tons of high explosive (a truckload) a kilometer away giving off a
one-second burst of heat and light that rivals the sun.

Now, if our hypothetical star were to go supernova, its brightness would increase 100
billion times, or be equivalent to 350 billion tons of explosive a kilometer away. 350
billion tons translates to 350,000 megatons, or much more than all the energy in all the
earth's nuclear weapons. In other words, the planet would receive a blast of heat and
light equivalent to having every nuclear weapon on earth detonated at the same time a
kilometer away -- and this intensity would last for days. It is no exaggeration to say
that the planet would be vaporized.

Fortunately, the melancholy idea of a star going supernova and frying the life on its
planets is unlikely. The very massive stars that produce supernovae do not shine long
enough for life to evolve beyond the simplest forms, and many may not even last long
enough for planets to finish the accretion process.

Supernova Remnants and Pulsars

For many thousands of years, a supernova can be recognized by the expanding shell of
gas blasted off the star. Such a shell is called a supernova remnant. The former
star itself is often detectable as a pulsating radio source, or pulsar. Pulsars
emit tremendous bursts of energy in radio, visible light, and x-ray wavelengths. These
bursts appear to originate in small regions of the neutron star, perhaps due to matter
falling onto the neutron star. These pulses are extremely regular, ranging in period from
.001 second to a few seconds. The period of the pulses is the rotation period of the
neutron star. As the parent star collapses, its rotation speeds up, just as a pirouetting
skater speeds up by drawing in her arms. If the sun, with a diameter of 864,000 miles (1.3
million km) were to shrink to a netron star 10 miles (16 km) in diameter, it would shrink
to 1/86400 of its present size, and its rotation would speed up 86400 times. Instead of
rotating every 28 days, the neutron sun would rotate in 28 seconds.

Supernovas and New Stars

Supernova remnants often appear to be associated with areas of new star formation, and
it is very likely that a supernova triggers the formation of new stars. If the expanding
blast wave from a supernova strikes a cloud of interstellar gas and dust, it can compress
the cloud enough that parts of the cloud start to contract gravitationally. The supernova
debris will also mingle with the cloud, enriching it in heavy elements.

In normal stars, it is not possible to form large amounts of elements much heavier than
iron. Elements like lead, gold, and uranium can form only in supernovae, as far as we can
tell. The fact that we find these elements in the solar system is a sign that our sun is
perhaps a third-generation star. The sun is only a third as old as the milky way galaxy,
and there was time for many cycles of stellar birth and death before the sun formed. The
implications of this idea are profound. Every atom in us formed in a star billions of
years ago and many light-years away.

Black Holes

If the collapsed core of a supernova is more than 1.4 times as massive as the sun, it
will not form a neutron star. Instead, there is no known force that can halt the
gravitational collapse. The star will contract until its gravity is so immense that not
even light can escape. As far as we know, the star will contract until it becomes a point,
detectable only by its gravity. What happens to the matter in the star? We can only
speculate, but it is possible, according to some theories in physics, that the matter
might re-emerge somewhere else in space and time.

These bizarre objects, called black holes, are favorite topics of speculation
among science-fiction and popular magazine articles, but have any actually been detected?
Possibly some have. A black hole orbiting another star might draw matter from the
companion star. As the gas fell into the black hole, it would accelerate to enormous
velocities and emit intense x-rays. There are a few x-ray sources that are so massive that
they appear likely to be black holes.

Stars and Origin of the Chemical Elements

Where did atoms come from, and how did there come to be so many different kinds? We can
answer this question in quite a bit of detail, thanks to what we know about nuclear
reactions in particle accelerators, reactors, and nuclear bombs.

The sun gets its energy by fusing four hydrogen nuclei (protons) to make a helium
nucleus. This process is actually fairly complicated and proceeds in several steps. The
final nucleus contains four particles, or nucleons: two protons and two neutrons.

Beyond helium we encounter a bottleneck. If we try to add a neutron or proton to a
helium nucleus, the new nucleus disintegrates almost instantly. There is no nucleus with
five nucleons that lasts long enough to be a basis for heavier elements. Perhaps we can
fuse two helium nuclei together? But it turns out that there are no long-lasting nuclei
with eight nucleons either.

However, there is a way around this bottleneck. When stars collapse to form red giants,
the temperatures and pressures inside the star become high enough for three-way collisions
of helium nuclei to occur. These collisions are rare, to be sure, but they occur often
enough to form heavier elements in the quantities we observe. The product of these
collisions has 12 nucleons: 6 protons and 6 neutrons, and is a carbon nucleus.

What about lithium, beryllium, and boron, the elements between helium and carbon? These
atoms form only when collisions knock nucleons off of heavier nuclei, a process called spallation,
and they tend to be destroyed in the interiors of stars. Compared to carbon, they are rare
in the universe. Actually, it is probably a good thing that it is so hard to form heavy
atoms. If it were easier, stars might long ago have fused all their hydrogen to heavy
atoms and there would be no available energy left in the universe.

Once carbon forms, it serves as a base for building heavier atoms. Nucleons can be
added one-by-one, or by fusing more helium nuclei to an existing nucleus. Building
elements by fusing helium nuclei is a common process, and elements with even numbers of
protons in their nuclei are more abundant than atoms with odd numbers. But each heavier
nucleus takes more energy to make and gives less energy back, until at iron, with 26
protons, the process ends. Beyond iron, it takes more energy to make a nucleus than the
nuclear reaction gives back. Random collisions build some heavier nuclei, but the
abundance of elements drops off sharply after iron.

Very heavy atoms like gold are extremely rare in the universe, and seem to
form only in two ways. One way, the s-process (for slow), is the
addition of neutrons one by one to existing nuclei. Eventually, once enough
neutrons have been added, one of them breaks down to a proton plus an electron.
The electron is expelled, a process called beta-decay, and the atomic number of
the element increases by one. Since the process is slow, many short-lived
radioactive nuclei decay away before another neutron can be added to the
nucleus. The atoms that can form by the s-process are limited to a narrow band
of stable and long-lived radioactive nuclei, and the r-process ends with the
formation of lead. The r-process happens in certain red giant stars, and the
heavy elements are blown into space as the star loses its outer shells.

the fierce environment in the core of a supernova. Much of the light given off by a
supernova turns out to be due to the decay of radiocative nickel. In a supernova, there is
so much energy available that particles can pile onto nuclei at a tremendous rate. Atoms
at least as heavy as plutonium form this way, and probably atoms far heavier than that.
These atoms require more energy to form than our most powerful particle accelerators can
produce, but nuclear physics predicts that nuclei with about 110 protons might have very
long lifetimes, possibly long enough to be left over from the formation of the earth. Some
physicists have attempted to find such super-heavy elements in rocks, so far without
success.